Process for separating specific radioelements singly or multiply from a mixture of radioactive elements

ABSTRACT

A PROCESS FOR SELECTIVELY REMOVING RADIOACTIVE ISOTOPES OF NA, K, AND/OR CL FROM ACIDIC AQUEOUS SOLUTIONS WHEREIN THESE SOLUTIONS ARE ADMIXED WITH ORGANIC SOLVENTS AND PASSED OVER COLUMNS MADE UP OF INORGANIC SALTS HAVING NONADIOACTIVE FORMS OF THE ISOTOPES WHICH ARE TO BE REMOVED FROM THE SOLUTION.

United States Patent C) 3,629,132 PROCESS FOR SEPARATiNG SPECIFIC RADIO- ELEMENTS, SINGLY R MULTIPLY, FROM A MIXTURE 0F RADIOACTIVE ELEMENTS Constantine J. Maletskos, Gloucester, and Chung-Wai Tang, Cambridge, Mass., assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Feb. 13, 1969, Ser. No. 799,118 Int. Cl. C09k 3/00 US. Cl. 252301.1 R 3 Claims ABSTRACT OF THE DISCLOSURE A process for selectively removing radioactive isotopes of Na, K, and/ or Cl from acidic aqueous solutions wherein these solutions are admixed with organic solvents and passed over columns made up of inorganic salts having nonradioactive forms of the isotopes which are to be removed from the solution.

BACKGROUND OF THE INVENTION Prior art The art has long sought a method of determining the distribution and abundance of trace elements in biological tissue. The introduction of large volume, high-resolution Ge(Li) detectors in activiation analysis has brightened the hope of developing a fully instrumental and non-destructive method for trace analysis. Such a technique would be ideal for the study of trace element distribution in human tissues. For trace elements that can be neutron activated to y-emitting radioisotopes of long half lives (T /z l month), the technique has been successfully demonstrated on geological as gell as biological samples, waiting for the decay of short-lived interfering radio-activities when necessary.

Experiments with irradiated human tissue samples show that the techniques used for long-lived radioisotopes are unsuccessful when applied to short-lived (T /2 l2 weeks) radioisotopes or when it is desirable to measure the long-lived isotopes soon after irradiation. The principal culprit for this setback is the intense radioactivity of readily neutron-activated sodium, potassium and/or chlorine over that of the trace elements in tissue samples. This interference from these elements arises from the preponderance of these elements in tissues and from their nuclear properties and abundance of 7 rays, the combination of which factors prevents a purely instrumental analysis of the y rays from the trace elements.

The art has sought an eflicient, economic method of removing these interfering elements from samples in order to permit the advantageous use of instrumental activation analysis.

Additionally, the convenient isolation of radioelements singly from a radioactive mixture is very useful for the study of the nuclear decay scheme of the isolated radioisotope or for the measurement of the radioactive product of a neutron-activated enriched isotope used as a non-radio active tracer, such as K or Cr, especially in biological systems.

SUMMARY OF THE INVENTION The invention described herein was made in the course of, or under a contract with the United States Atomic Energy Commission.

We have discovered a process for selectively removing specific radioelements from a mixture of radioelements in an aqueous solution by acidifying the aqueous solution up to 40 volume percent, based upon the volume of the aqueous solution, dispersing the aqueous solution in an elutant, said elutant having a boiling point of less than C.; passing the eluate containing the radioelements through a column, said column being composed of an inorganic salt having as an element thereof a nonradioactive form of the radioactive isotope which is sought to be removed from the mixture, said inorganic salt being substantially insoluble in said organic elutant; thereafter recovering he radioactive mixture from the elutant free of the radioelement removed by the column.

It is to be understood that our novel process can be employed to remove both a single radioactive element or a mixture of different radioactive elements at the same time providing that the column contains a non-radioactive form of each element which is desired to be removed from the system.

In various experiments, this process has been successfully applied to the specific removal of K, Na and Cl from irradiated tissue samples, and of Na and Ba from solutions of mixed radioactive tracers.

DETAILED DESCRIPTION OF THE INVENTION In the practice of our invention, the techniques are somewhat similar to conventional ion-exchange. The colurnns containing the inorganic salt can be constructed in the standard manner. The duration of the passage of the eluate depends on factors such as the size of the column and nozzle, the amount and grain size of the salt used in the column, the composition of the eluate, the height of the eluate above the salt column, etc. Usually, the slower the flow rate, the better the decontamination or effectiveness of separation. The various factors can be varied according to the requirements of the individual experiments. The greater the amount of the radioelement to be removed, the more salt is required on the column. In many cases, it may be necessary to wash the column with the elutant in order to recover all of the elements not to be removed by the column. At times, it may also be necessary to complex some of the elements that may undesirably be retained on the column with organic chelating agents like dithizone, acetylacetone, etc.

The choice of salt clearly depends on the particular element or elements to be removed and on the nature of the original mixture. For example, radioactive isotopes of sodium, chlorine, and potassium contained in a mixture of other radioelements can easily be removed by utilizing a column containing a mixture of NaCl and KCl. In such cases, an inter-mixture of the salts or segregation of the salts into separable zones in an individual column can be employed.

The organic eluting agents useful in our invention can be of any organic solvent which is substantially miscible or can be made miscible with the aqueous solution containing the mixture of radioelements from which the radioactive isotopes are to be separated. The choice of elutant, of course, is quite dependent on the inorganic salt selected for use in the column since the salt must be substantially insolvent in the elutant, Exemplary of the type solvents found useful in our invention are acetone, butanol, diethyl ether and mixtures thereof.

Any inorganic salt having the requisite ions in its makeup can be employed. For the removal of Na and K, we have chosen sodium chloride and potassium chloride, utilizing acetone as the elutant. This choice is dictated mainly by the fact the NaCl and KCl are less expensive than salts like NaBr and KBr, which have proven to be very eifective as well. Furthermore, NaBr and KBr will also remove radioactive bromine, Br, which may not be a desirable objective.

A simplified illustration of the practice of our invention is given as follows: one ml. of the aqueous solution (tissue or tracer solution) is mixed thoroughly with 9 ml. of acetone, and 5 drops of concentrated HCl are added to the mixture. The solution is then eluted through a NaCl column, 10 cm. long x 0.5 cm. [.D. Decontamination factors greater than 10 with either *Na or Na tracers have been achieved. Because the elutant has a relatively low boiling point, it can easily be concentrated by evaporation over low heat.

Should other elements be undesirably retained on the column, they can be recovered by further elution of the column with the elutant or by adding chelating agents to the solution prior to its passage through the column.

EXAMPLE Materials and reagents NaCl crystals, reagent or USP. grade KCI crystals, reagent or U.S.P. grade Methyl isobutyl ketone (hexone), A.C.S. grade Acetone, N.F. or A.C.S. grade Concentrated HCl, reagent or technical grade Concentrated HNO reagent or technical grade Concentrated HC1O (70%) reagent, A.C.S. grade Formaldehyde HCHO, N.F. grade 120-cm. long glass tubing 7 mm. OD. and 5 mm. ID. 90-cm. glass rod of 4 mm. diameter Glass wool 5% inch disposable capillary Pasteur pipettes and rubber bulbs 50-ml. disposable, polypropylene beakers SO-ml. and ISO-ml. Pyrex glass beakers 2-pint screw-capped glass jars China marker Burette stand and clamp Ruler Equipment Mortar and pestle (hand or mechanical) or ball mill Standard laboratory oven Desiccator with Drierite Sieves (230 mesh and 325 mesh) Electric hot plate Radiation protection instruments, etc.

Ge(Li) detector, l024channel analyzer, plotter and assorted electronic equipment.

Preparation of columns Reagent and USP. grade NaCl crystals were dried in an oven at 110 C. for at least 5 hours. The dried crystals were then ground into a fine powder (using a mortar and pestle or a ball mill). Powder sifted through 230 and retained on 325 US. Standard mesh sieves was placed in screw-capped jars and stored in a desiccator with Drierite until used.

The same procedure was followed for U.S.P. or reagent grade KCl crystals.

For the column, a 60-cm. long glass tubing 7 mm. OD. and 5 mm. ID. with one end drawn to a thin nozzle was used. A small wad of glass wool was inserted into the tube and rammed tight against the nozzle end with a long, smaller-diameter glass rod. The glass tube was held vertically in a burette clamp with the nozzle end down. Distilled water was run through the columns, followed by acetone, to wash and dry the glass tube.

Using a China marker, the l0-cm. and 20-cm. distances from the glass wool plug were marked off on the column. NaCl powder of 230-325 mesh was slurried with acetone in a 50 ml. disposable polypropylene beaker. Using a disposable capillary Pasteur pipette, the NaCl-acetone slurry was introduced into the column until enough NaCl settled to the lO-cm. level by gravity. NaCl powder sticking to the inside walls of the column was washed down with acetone. The column was tapped lightly with a pencil or ruler so that the top of the salt bed settled down smoothly. Excessive tapping must be avoided so that the salt crystals will not be packed too tightly to impede reasonable fiow rate. When the level of the NaCl dropped below 10 cm., more NaCl-acetone slurry was added until the top settled approximately at the l0cm. mark. The acetone in the column was never allowed to drain out completely in order to prevent the formation of channels in the salt bed.

In a similar manner, the KCl powder of 230-325 mesh was packed into the column from the IO-cm. mark to the 20cm. mark. The result was a column of two different zones, a 10-cm. long KCl bed on top of a lO-cm. long NaCl bed.

The acetone that drained down the column during the packing of the salt was collected in a beaker and saved for use in preparing the eluate.

Digestion of tissue A 50l00 mg. piece of irradiated, lyophilized, human lung tissue was placed in a lSO-ml. Pyrex beaker and soaked in 7 ml. of cone. HNO and 3 ml. of cone. HClO (All the tissue digestion procedures were conducted in a hood.) The beaker was then heated over a hot plate until the tissue was completely digested and the volume of the solution had boiled down to -l ml. The beaker was removed from the hot plate and -1 ml. of N.F. grade formaldehyde was added to the solution, and the solution was boiled again until the residual solution was -1 ml. The digested tissue solution was allowed to cool.

Preparation of eluate To the beaker containing the tissue solution, 5 drops of cone. HCl, 2 ml. of hexone, and 7 ml. of acetone collected from the column packing were added and mixed thoroughly. The mixture was now ready for elution. Five drops of the solution were withdrawn and placed in a small polyethylene vial to obtain a Ge(Li) spectrum of the sample before the Na and K removal.

Elution procedure The acetone in the column was allowed to drain down to the top of the KCl bed. With a disposable capillary Pasteur pipette, the tissue eluate was introduced onto the column. Since the column could not hold the full 10 ml. of solution, the rest of the solution was added after part of the eluate had fiowed through. The eluant was collected in a SO-ml. Pyrex beaker. As the eluate level reached the top of the KC] in the column, -10 ml. of plain acetone were eluted through as wash and collected with the sample. The eluant was counted with a Ge(Li) detector to determine the degree of Na and K decontamination. When the -ray spectrum showed that the decontamination was insufficient to show adequately the trace-element photopeaks in the tissue, the eluant was passed through another new KCl-NaCl column similarly prepared as above. This procedure can be repeated until sufficient decontamination is achieved. Each column must be thoroughly washed with acetone after eluting the sample, and the wash included with the main eluant for counting. For the lung tissue sample described above, a second elution through a new column was sufficient.

If necessary, for counting or for subsequent chemical operations, the acetone in the eluant may be concentrated by evaporation or distillation in low heat.

In this particular example we have achieved a decontamination factor of greater than 10 for Na.

For purposes of this application the term elutant means solvent medium used in the elution, eluate means the solution to be passed through the column and eluant means the solution emerging from the column.

What is claimed is:

1. A process for selectively removing radioactive sodium and potassium from mixtures of radioisotopes comprising:

(a) dispersing the mixture containing the radioactive sodium and potassium in an acidic aqueous solution, said solution containing up to 40 volume percent of an inorganic acid;

(b) dispersing approximately 1 part of the acidic aqueous solution in approximately 9 parts of a water miscible organic elutant, said elutant having a boiling point of less than 100 C.;

(c) passing the eluate containing the radioactive 5 sodium and potassium over a column of solid nonradioactive inorganic salt, having as an element thereof a nonradioactive form of sodium and potassium, said inorganic salt being substantially insoluble in the organic eluant. 2. A process in accordance with claim 1 wherein the radioactive element to be removed is a radioactive isotope of sodium.

3. A process in accordance with claim 2 wherein said column contains sodium chloride.

References Cited UNITED STATES PATENTS 3,251,646 5/1966 Alon et al. 23312 3,357,800 12/1967 Gaska 23-312 LELAND A. SEBASTIAN, Examiner 10 P. A. NELSON, Assistant Examiner U.S. Cl. X.R.

23--3l2 AH, 337; 424-1 

